This invention relates generally to an apparatus for processing electrical signals and in particular to an apparatus for aligning brushes of a dynamoelectric machine to minimize brush arcing.
In dynamoelectric machines such as dc generators brushes are used to conduct current to and from collector slip rings or commutators attached to a rotor. Typically, the brushes are mounted in a stationary brushholder assembly or rigging such as is shown, for a large generator, in U.S. Pat. No. 3,864,803 to Ohmstedt et al. which is also assigned to the same assignee as the present invention. This assembly supports the brushes and allows them to ride stably on the slip ring surface of the rotor. Although the brushholder rigging is held in fixed position during operation of the dynamoelectric machine, a small amount of rotational adjustment is often provided to permit varying of the commutation angle of the brushes for optimum performance of the dynamoelectric machine. In particular, the rigging should be set at a position such that arcing of the brushes during operation of the machine is avoided since, as is well known, arcing between the brushes and rotor slip ring results in electrical losses and can lead to damaged rotor slip rings, brushes, and brushholder riggings, forcing outages and repairs. For most machines, the total arc length through which the brushholder rigging may be rotated without resulting in brush arcing is quite small, for example, less than an inch even for large generators of power ratings of up to 3,000 kilowatts. The optimum rigging position within this narrow, arc-free region, which is known as the "black band" because of the absence of visible brush arcing when the machine is operated in this region, is normally determined for a generator prior to shipment by means of tests known as "buck" and "boost" tests. During these tests, which may be conducted at part generator load or at full load, current is applied to the generator field either in excess of that normally furnished by the excitation system (boost current) or less than that normally furnished (buck current), and output signals from the brushes are measured by a meter. In general, the apparatus output signals rise with increasing magnitude of buck or boost current applied, defining, when output signal is plotted against applied current, a curve which is approximately parabolic in shape whose minimum value of output signal may occur at a value of applied current on either side of the electrical neutral, the point at which only normal excitation current (no buck or boost current) is being applied to the generator field. The two end points of the parabolic-shaped curve are defined by levels of buck and boost current. An output signal at which the brushes begin arcing is determined optically or by means of a brush arc monitor such as described in U.S. Pat. No. 4,163,227 to Sawada et al. and which is also assigned to the same assignee as the present invention. For each brushholder rigging setting, a different parabolic-shaped curve may be generated, and the optimum rigging setting is determined as that for which minimum output signal occurs at or displaced a prescribed increment of current from, electrical neutral, and also as that which has substantially the maximum arc-free range of buck and boost currents. These criteria assure minimum brush arcing during normal operation of the dynamoelectric machine and thus long brush and slip ring lives.
Prior to the present invention, the above-described output signals from the brushes of the dynamo-electric machine being aligned were fed to an apparatus described in U.S. Pat. No. 4,204,156 issued May 20, 1980 to Sawada et al. and assigned to the same assignee as the present invention. Although very useful in the alignment of generator brushes, the apparatus described in this patent possesses several undesirable limitations. First, because of the nature of the signal processing which occurs, the output null voltage is higher than is necessary. Because this is so, the dynamic swing of the output voltage is significantly reduced. Second, the sharpness of the above-described parabolic-shaped curve is less than desirable, indicating an undesirable level of sensitivity for the output voltage as a function of the level of boost or buck current. In particular, this design permits passage of a level of low-frequency voltage which adversely affects the null point level and the alignment monitor sensitivity.
Accordingly, it is an object of the present invention to decrease the operator time involved in the alignment of generators and motors.
It is a further object of the present invention to provide brush rigging alignment with a decreased need for plotting a large number of sample points. It is also an object of the present invention to provide an alignment monitor having greater accuracy and stability with a greater reduction of noise.